The invention relates to a method for supplying binder into a coating tank for electrophoretic powder coating, and its apparatus.
Recently, an electrophoretic powder coating, which is generally called an EPC, has been developed and used widely. The EPC is of cationic type electrophoretic coating in which powder is used for the major component for layer formation and cataphoresis, electrolysis, electroendosmosis and electroextraction cooperate to form a coating layer on the surface of an article. The major difference of the EPC from the conventional anionic type electrophoretic coating is to use such a coating material in which insoluble powder is suspended in diluted solution of water soluble cationic binder and to apply a DC voltage in such a manner that an article dipped in plating bath liquid serves as a cathode and an electrode dipped in the same as an anode. Thus the coating layer formed on the surface of the article is composed of cationic binder and powder. The EPC has the following advantages: (i) The thick coating layer is formed for a relatively short time, e.g. for several seconds. (ii) The thickness of coating layer is easily adjustable. (iii) No elution of the metal from the article is occurs, because the coating is excuted by means of cationic type electrophoretic coating. (iv) The coating layer is excellent in tightness and corrosion proof. (v) Coating work is hygienic one with no harm by solvent and powder explosion by dust. (vi) Powder is collected almost completely.
In EPC, two coating components, i.e. cationic binder and powder, in the plating bath liquid are gradually consumed as the coating work progresses. The reduction of these components under a given value results in deterioration of coating quality. Therefore, the reduced component or components must be supplied in the course of coating work. The supply of the reduced component; however, encounters many problems and its operation is complex. For example, in the binder supply, mother binder liquid can not be directly mixed with the plating bath liquid. That is, if the mother binder liquid is directly poured into the bath liquid of which the powder concentration is about the low limit value ensuring proper mixing, the mother binder liquid is not uniformly dispersed into the bath liquid and thus the binder concentration becomes never uniform in the bath liquid.
Many attempts to solve such disadvantages have been made but the results of them have been all insufficient.